1. Field of the Invention
The present invention relates generally to heat transfer tubes for evaporators in air conditioning and refrigeration systems, more particularly, to a heat transfer tube that has an outer wall surface formed therein with double cavity.
2. Description of the Related Art
Many fields, such as refrigeration, air conditioning, process engineering, petrochemical processing, and energy source and power engineering, relate to evaporating and boiling of a liquid on an outer wall surface of a tube. Especially in evaporators used in air conditioning and refrigeration systems, a thermal resistance of boiling heat transfer in the case that a refrigerant is boiling on an outer wall surface of a tube corresponds substantially to and even larger than that of the forced convection in the tube. Therefore, it can significantly improve the heat transfer performance of the evaporator to enhance the boiling heat transfer on the outer wall surface of the tube.
It was found from the study on the mechanism of the nucleate boiling that the boiling of a liquid requires the existence of nucleation sites for evaporating. For a heating surface with a given superheating temperature, only when a radius of a nucleation site for evaporating is larger than a minimum radius required for the growing of a vapor bubble, the vapor bubble can grow up so that the nucleate boiling process can be performed. Cavities formed from grooves and cracks in the heating surface most probably become nucleation sites for evaporating. During boiling, after the vapor bubbles grow up and break away from the cavities, as it is difficult for a portion of steam retained by the cavities to be completely expelled by a liquid flowing towards the cavities due to the action of surface tension of the liquid, the cavities become new nucleation sites again. New vapor bubbles grow from the new nucleation sites so that the boiling process constantly continues. Therefore, it is critical to form many nucleation sites on the heating surface in order to enhance the heat transfer of the nucleate boiling.
Since 1970s, many developments for the enhancement of the performance of boiling heat transfer surfaces have been carried out based on formation of porous structure on the heating surface, which can be found from a lot of references. For example, Chinese Patents Nos. 2257376Y and 2662187Y disclose a heat transfer tube for an evaporator, of which an outer surface is formed with helical fins with tops pressed in a T shape so as to constitute channel structure; Chinese Patents Nos. CN1090759C and CN2557913Y disclose a heat transfer tube, of which an outer surface is formed with helical fins with inclined teeth uniformly arranged circumferentially, and a cavity structure is formed by pressing the fins so that tops of the fins extend towards both sides thereof; China Application Publication No. 1366170A discloses a heat transfer tube, of which an outer surface is formed with fins by machining, and secondary channels are formed at bottoms of primary channels between the fins; Chinese Patent No. 1100517A discloses a heat transfer tube, in which fins on an outer surface of the heat transfer tube are pressed to be inclined towards one side, and then notches are impressed into the shoulder of the fins in order to constitute a cavity structure on the outer surface of the heat transfer tube; Chinese Patent No. 2572324Y discloses a heat transfer tube for an evaporator, of which an outer surface is formed with helical fins with sawtooth shape, and then inclined notches are impressed into tops of sawtooth in order to manufacture a cavity structure on the outer surface of the heat transfer tube. The outer wall surfaces, which are also called outer fin structure, of the heat transfer tubes disclosed in the above references have a common structural feature that the heat transfer tubes are disposed with channels or cavities with slightly small openings to constitute nucleation sites for evaporating, so as to enhance the boiling heat transfer. With the further study on the mechanism of the nucleate boiling, however, it has been found that after the vapor bubbles are formed, evaporation of liquid micro layers between the wall and the bottoms of the vapor bubbles plays an important role and even a dominant role in the growing process of the vapor bubbles. The experiment on the boiling heat transfer in a lower liquid level shows that after the liquid level is lower than a critical value which is less than two times the diameter of a vapor bubble, when a previous vapor bubble escapes the heat surface to ascend, it can not immediately break away from the heating surface since it is subject to the suppression of a liquid surface. When a next vapor bubble grows, it is oppressed by the previous vapor bubble so as to grow in hemisphere shape. Therefore, a liquid micro layer below the vapor bubble has a large evaporating area, thereby significantly improving boiling heat transfer coefficient. It was demonstrated from the experiment that since a liquid micro layer below the vapor bubble has a thickness of the order of magnitude of about 1 micrometer, so that it has a much small thermal resistance. If the area of the liquid micro layer of the vapor bubble bottom is enlarged or the duration of the liquid micro layer of the vapor bubble bottom is prolonged, the boiling heat transfer will be enhanced.
However, in the disclosed references, the fins on the outer wall surfaces of the heat transfer tubes for evaporators can not achieve such an effect which improves the boiling heat transfer coefficient and boiling heat transfer significantly, as has been demonstrated by the above experiment. Moreover, the heat transfer tubes are heavy in weight, thereby wasting raw material.